The presently disclosed embodiments are directed to the preparation and processing of an imaging member to achieve physically and mechanically improved performance for use in electrostatography. More particularly, the embodiments pertain to the development of a structurally simplified flexible electrophotographic imaging member without the need of an anticurl back coating layer and a post treatment process for the member service life extension in the field.
In electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Flexible electrostatographic imaging members are well known in the art. Typical flexible electrostatographic imaging members include, for example: (1) electrophotographic imaging member belts (belt photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper. The flexible electrostatographic imaging members may be seamless or seamed belts; and seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. Typical electrophotographic imaging member belts include a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anticurl back coating coated onto the opposite side of the substrate layer. A typical electrographic imaging member belt does, however, have a more simple material structure; it includes a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Although the scope of the present embodiments covers the preparation of all types of flexible electrostatographic imaging members, however for reason of simplicity, the discussion hereinafter will focus and be represented only on flexible electrophotographic imaging members.
Electrophotographic flexible imaging members may include a photoconductive layer including a single layer or composite layers. Since typical flexible electrophotographic imaging members exhibit undesirable upward imaging member curling, an anti-curl back coating, applied to the backside, is required to balance the curl. Thus, the application of anti-curl back coating is needed to provide the appropriate imaging member belt with desirable flatness.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge generating layer is sandwiched between the outermost exposed charge transport layer and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The charge generating layer then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the charge transport layer. In the alternate case when the charge transport layer is sandwiched between the charge generating layer and the conductive layer, the outer surface of the charge generating layer is charged positively while conductive layer is charged negatively and the holes are injected through from the charge generating layer to the charge transport layer. The charge transport layer should be able to transport the holes with as little trapping of charge as possible. In flexible imaging member belt such as photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members should be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers, and may optionally include an overcoat layer over the imaging layer(s) to provide abrasion/wear protection. In such a photoreceptor, it does usually further comprise an anticurl back coating layer on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, charge generating layer, charge transport layer, and other layers.
Typical negatively-charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer. The charge transport layer is usually the last layer, or the outermost layer, to be coated and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 120° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing solution application of the charge transport layer coating and through drying/cooling process, upward curling of the multilayered photoreceptor is observed. This upward curling is a consequence of thermal contraction mismatch between the charge transport layer and the substrate support. Since the charge transport layer in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the charge transport layer does therefore have a larger dimensional shrinkage than that of the substrate support as the imaging member web stock cools down to ambient room temperature. The exhibition of imaging member curling after completion of charge transport layer coating is due to the consequence of the heating/cooling processing step, according to the mechanism: (1) as the web stock carrying the wet applied charge transport layer is dried at elevated temperature, dimensional contraction does occur when the wet charge transport layer coating is losing its solvent during 120° C. elevated temperature drying, but at 120° C. the charge transport layer remains as a viscous flowing liquid after losing its solvent. Since its glass transition temperature (Tg) is at 85° C., the charge transport layer after losing of solvent will flow to re-adjust itself, release internal stress, and maintain its dimension stability; (2) as the charge transport layer now in the viscous liquid state is cooling down further and reaching its glass transition temperature (Tg) at 85° C., the charge transport layer instantaneously solidifies and adheres to the charge generating layer because it has then transformed itself from being a viscous liquid into a solid layer at its Tg; and (3) eventual cooling down the solid charge transport layer of the imaging member web from 85° C. down to 25° C. room ambient will then cause the charge transport layer to contract more than the substrate support since it has about 3.7 times greater thermal coefficient of dimensional contraction than that of the substrate support. This differential in dimensional contraction results in tension strain built-up in the charge transport layer which therefore, at this instant, pulls the imaging member upward to exhibit curling. If unrestrained at this point, the imaging member web stock will spontaneously curl upwardly into a 1.5-inch tube. To offset the curling, an anticurl back coating is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness.
Although it is necessary to have the anticurl backing layer to complete a typical imaging member web stock material package, nonetheless the application of anticurl backing layer onto the backside of the substrate support (for counter-acting the upward curling and render the imaging member web stock flatness) has caused the charge transport layer to instantaneously build up an internal tension strain of about 0.28% in its material matrix. After converting the web stock into a seamed imaging member belt, the internal built-in strain in the outermost charge transport layer is then cumulatively adding onto each belt bending induced strain as the belt flexes over a variety of belt module support rollers during dynamic belt cyclic function in a machine. The consequence of this compounding strain effect has been found to cause early onset of imaging member belt fatigue charge transport layer cracking problem; the emergence of cracking in the charge transport layer is then led to the manifestation of undesirable printout defects in the image receiving copies.
Moreover, various imaging member belt functioning deficiencies associated with the common anticurl back coating formulations used in a typical conventional imaging member belt have also been observed under a normal machine functioning condition in the field; they are, for example, exhibition of anticurl back coating wear and its propensity to cause electrostatic charging-up are the frequently seen problems to prematurely cut short the service life of a belt. Anticurl back coating wear under the normal imaging member belt machine operational conditions reduces the anticurl back coating thickness, causing the lost of its ability to fully counteract the curl as reflected in exhibition of gradual imaging member belt curling up in the field. Curling is undesirable during imaging belt function because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, causing non-uniform charging. In addition, developer applicators and the like, during the electrophotographic imaging process, may all adversely affect the quality of the ultimate developed images. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper. Since the anticurl back coating is also an outermost exposed bottom layer and has high surface contact friction when it slides over the machine subsystems of belt support module, such as rollers, stationary belt guiding components, and backer bars, during dynamic belt cyclic function, these mechanical sliding interactions against the belt support module components not only exacerbate anticurl back coating wear, it does also cause the relatively rapid wearing away of the anti-curl produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Moreover, anticurl back coating abrasion/scratch damage does also produce unbalance forces generation between the charge transport layer and the anticurl back coating to cause micro belt ripples formation during electrophotographic imaging processes, resulting in streak line print defects in output copies to deleteriously impact image printout quality and shorten the imaging member belt functional life.
Undesirably, high contact friction of the anticurl back coating against machine subsystems is further seen to cause the development of electrostatic charge built-up problem. In other machines the electrostatic charge builds up due to contact friction between the anti-curl layer and the backer bars increases the friction and thus requires higher torque to pull the belts. In full color machines with 10 pitches this can be extremely high due to large number of backer bars used. At times, one has to use two drive rollers rather than one which are to be coordinated electronically precisely to keep any possibility of sagging. Static charge built-up in anticurl back coating increases belt drive torque, in some instances, has also been found to result in absolute belt stalling. In other cases, the electrostatic charge build up can be so high as to cause sparking. Additionally, a further short coming seen is that the cumulative deposition of anticurl back coating wear debris onto the backer bars does give rise to undesirable defect print marks formed on copies because each debris deposit become a surface protrusion point on the backer bar and locally forces the imaging member belt upwardly to interferes with the toner image development process. On other occasions, the anticurl back coating wear debris accumulation on the backer bars does gradually increase the dynamic contact friction between these two interacting surfaces of anticurl back coating and backer bar, interfering with the duty cycle of the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases.
Therefore, each of the anticurl back coating failures disclosed in preceding does require frequent costly belt replacement in the field. It is also important to point out that an electrophotographic imaging member belt prepared to require an anticurl back coating for flatness does have more than the above list of problems, they do indeed incur additional material and labor cost impact to imaging member production process. Although many attempts have been made to overcome these problems in earlier prior art works, nonetheless the solution of one problem has often seen to lead to the creation of additional problems. In summary, electrophotographic imaging members comprising a supporting substrate, having a conductive surface on one side, coated over with at least one photoconductive layer (such as the outermost charge transport layer) and coated on the other side of the supporting substrate with a conventional anticurl back coating that does exhibit deficiencies which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers. While the above mentioned electrophotographic imaging members may be suitable or limited for their intended purposes, further improvement on these imaging members are required. For example, there continues to be the need for improvements in such systems, particularly for an imaging member belt that has sufficiently flatness, superb wear resistance, nil or no wear debris, eliminates electrostatic charge build-up problem, extended charge transport layer cracking, and defects free printout copies even in larger printing apparatuses. With many of the above mentioned shortcomings and problems associated with electrophotographic imaging members having an anticurl back coating now understood, therefore there is an urgent need to resolve these issues through the development of a methodology for fabricating imaging members that produce improve function and meet future machine imaging member belt life extension need. In the present disclosure, a charge transport layer material reformulation method and process of making a flexible imaging member free of the mentioned deficiencies have been identified and demonstrated through the preparation of anticurl back coating free imaging member. The present disclosure of the formulation an improved curl-free imaging member without the need of a conventional anticurl back coating in combination of a post imaging member treatment process to effect abrasion/wear failure suppression and reduction or free of the built-in internal tension strain in the charge transport layer for cracking life extension will be fully described in detail in the following.
Conventional photoreceptors are disclosed in the following patents, a number of which describe the presence of light scattering particles in the undercoat layers: Yu, U.S. Pat. No. 5,660,961; Yu, U.S. Pat. No. 5,215,839; and Katayama et al., U.S. Pat. No. 5,958,638. The term “photoreceptor” or “photoconductor” is generally used interchangeably with the terms “imaging member.” The term “electrostatographic” includes “electrophotographic” and “xerographic.” The terms “charge transport molecule” are generally used interchangeably with the terms “hole transport molecule.”
Yu, U.S. Pat. No. 6,183,921, issued on Feb. 6, 2001, discloses a crack resistant, curl free electrophotographic imaging member includes a charge transport layer comprising an active charge transporting polymeric teraaryl-substituted biphenyldiamine and a plasticizer.
Yu, U.S. Pat. No. 6,660,441, issued on Dec. 9, 2003, discloses an electrophotographic imaging member having a substrate support material which eliminates the need of an anticurl backing layer, a substrate support layer and a charge transport layer having a thermal contraction coefficient difference in the range of from about −2×10−5/° C. to about +2×10−5/° C., a substrate support material having a glass transition temperature (Tg) of at least 100° C., wherein the substrate support material is not susceptible to the attack from the charge transport layer coating solution solvent and wherein the substrate support material is represented by two specifically selected polyimides.
Yu, U.S. Pat. No. 6,743,390, issued on Jun. 1, 2004, discloses a method of treating a flexible multi-layer member exhibiting a glass transition temperature and including a surface layer, the method composed of: moving the member through a member path including a contact zone defined by contact of the member with an arcuate surface including a curved contact zone region; a pre-contact member path before the contact zone; and a post contact member path after the contact zone; and a post-contact member path after the contact zone; heating sequentially each portion of the surface layer such that each of the heated surface layer portions has a temperature above the glass transition temperature while in curved contact zone region; and cooling sequentially each of the heated surface later portions while in the contact zone such that the temperature of each of the heated surface layer portions falls to below the glass transition temperature prior to each of the heated surface layer portions exiting the curved contact zone region, thereby defining a cooling region, wherein the heating is accomplished in a heating region en compassing ant part or all of the zone outside the cooling region and a portion of the pre-contact member path adjacent the contact zone.
In U.S. Pat. No. 7,413,835 issued on Aug. 19, 2008, it discloses an electrophotographic imaging member having a thermoplastic charge transport layer, a polycarbonate polymer binder, a particulate dispersion, and a high boiler compatible liquid. The disclosed charge transport layer exhibits enhanced wear resistance, excellent photoelectrical properties, and good print quality.
In U.S. Pat. No. 7,455,802, there is disclosed a stress/strain relief process for a flexible, multilayered web stock including at least one layer to be treated, the at least one layer to be treated having a coefficient of thermal expansion significantly different from a coefficient of thermal expansion of another layer; passing the multilayered web stock over and in contact with a first wrinkle-reducing roller that spontaneously creates transverse tension stress in the at least one layer to be treated; heating at the at least one layer to be treated above a glass transition temperature Tg of the at least one layer to be treated to thereby create a heated portion of the at least one layer to be treated, a portion of the web stock in proximity to the heated portion of the at least one layer to be treated thereby becoming a heated portion of the web stock; including curvature in the heated portion of the web stock; and cooling the heated portion of the web stock at said curvature.
In U.S. Publication No. 2006/0099525, filed on Nov. 5, 2004, entitled “Imaging Member” to Yu et al., there is disclosed an imaging member formulated with a liquid carbonate. The imaging electrostatographic member exhibits improved service life.
In U.S. Publication No. 2006/0151922, filed on Jan. 10, 2005, entitled “Apparatus and Process for Treating a Flexible Imaging Member Stock” to Yu et al., there is disclosed a process for producing a stress relief electrophotographic imaging member we stock comprising: providing a multilayered imaging member web stock including at least one layer to be treated, the at least one layer to be treated having a coefficient of thermal expansion significantly differing from a coefficient of thermal expansion of another layer; passing the multilayered web stock over and making contact with a circular treatment tube having a outer concave arcuate circumferential surface that spontaneously creates a transverse web stock stretching force to offset the ripple causing transversal compression force in the at least one layer to be treated; heating at least one layer to be treated above the glass transition temperature (Tg) of the at least one layer to be treated to thereby create a heated portion of the at least one layer to be treated, a portion of the web stock in proximity to the heated portion of the at least one layer to be treated thereby becoming a heated portion of the web stock; including curvature conformance in the heated portion of the web stock; and cooling the heated portion of the web stock at said curvature to a temperature below the Tg of the layer.